Monday, October 8, 2007

Skink Diversification

Labmates Dan Rabosky and Amanda Talaba recently published a paper on Dan's grad work on explosive skink radiations in the Australian Outback. I posted this Cornell Chronicle article on the Repashy forums, generating some interested questions. I convinced Dan to respond, to enlight us with more details of his research. The exchange is replicated below.

The journal article:

Daniel L. Rabosky, SC Donnellan, AL Talaba, IJ Lovette. 2007. Exceptional among-lineage variation in diversification rates during the radiation of Australia's most diverse vertebrate clade. Proceedings of the Royal Society B. Online date: Sept 18, 2007.

The Cornell Chronicle article (source):

Cornell Chronicle Online

Sept. 19, 2007
Australian skinks show researchers why some species have 'explosively' diversified over time

A new study of finger-sized Australian lizards sheds light on one of the most striking yet largely unexplained patterns in nature: Why is it that some groups of animals have evolved into hundreds, even thousands of species, while other groups include only a few?

A leopard skink sits on a finger
Dan Rabosky
Skinks have incredible species diversity throughout the Australian deserts. In fact, this group contains so many species that most do not even have a common English name. Pictured is a leopard skink.

The study takes a look at Australia's most diverse group of vertebrates -- more than 252 species of lizards called skinks. Researchers at the Cornell Lab of Ornithology have found evidence that the "drying up" of Australia over the past 20 million years triggered this explosive diversification. The results were published in the Sept. 19 online edition of the Proceedings of the Royal Society B.

Lead author Dan Rabosky, a Cornell graduate student, spent many months in the remote Australian outback, trapping skinks as they skittered from one prickly clump of grass to another. By documenting where the various skink species live and using their DNA to define their evolutionary tree, he found that the groups with the most species are those that live in the driest parts of Australia.

"There's something about colonizing the desert that caused these skinks to diversify at an incredibly high rate," said Rabosky.

An unusual finding of this study is that these skinks upend the usual pattern of species diversity found in other parts of the world.

"We typically think of lush tropical rainforests as being the world's major centers of diversity," said co-author Irby Lovette, director of the laboratory's Fuller Evolutionary Biology Program. "With the skinks, just the opposite has happened: The rainforest skinks in Australia have much lower diversity, and a lot of the evolutionary 'action' in this system is taking place in the deserts."

Over the last 20 million years, most of Australia has changed from humid and tropical to bone-dry desert. "Living in the desert is stressful for animals that are adapted for wetter habitats," said Rabosky. "But somewhere in the distant past, a few skinks developed the ability to survive in their increasingly arid world."

It is the descendents of these few early desert colonists that evolved into amazingly large numbers of skink species, say the researchers.

"Australian skinks are really fascinating," Rabosky said. "Two groups in particular have gone evolutionarily crazy, each splitting into as many as 100 different species. In contrast to skinks on other continents, and even some other groups in Australia, the diversity of these particular groups has really exploded."

Rabosky's study included skinks with spots, stripes, four legs, two legs and no legs. Rabosky said that there are at least 252 species of these lizards living down under, and probably many more that remain to be discovered.

The evolution of these skinks, he said, mirrors that of many groups of organisms -- from grasses and beetles to humans and our relatives -- in which some groups have spectacular diversity and others a paucity of species.

"For me as a scientist," said Rabosky, "one of the great things about skinks is that there are just so darn many species, making the patterns in their diversity really clear."

Pat Leonard is a staff writer at Cornell's Lab of Ornithology.

Dan's Responses:

Originally Posted by Salzy
> "That's definitely interesting Nick. What is he doing to continue his
> research? Is he doing all the genetics testing now and then going
> to go back? Sounds like it would be an incredible project to work on."

My work here in the US involves lots of genetic work to reconstruct the evolutionary history of Australian skinks. But the main reason I'm interested in an evolutionary tree is so that I can test different potential explanations for what caused the explosion of Australian skink species over the past 20 million years. I do a bit of fieldwork in Australia - mainly to collect ecological data (habitat, etc) - to better understand how species evolve to do "new" things (like specialize on new habitats or feed on new prey resources). It is a fun project! There are more species of lizards occurring together in the deserts of interior Australia than anywhere else on Earth. At my field research sites, I have seen 40 species of lizards in a single day, which is pretty tough to replicate anywhere else...

Originally Posted by ominously
> "So what was the causative factor? What was the evidence that
> demonstrated
> this explosive diversification? What is the genetic basis for the
> theory?

Tough to say for sure what caused it, but there are some intriguing leads. The evidence for the explosive diversification is that we've built an evolutionary tree of a number of skink species and used statistical techniques to explicitly quantify how rates of evolutionary diversification have changed over the past 25 million years or so (and thus were able to show that one group has undergone a massive increase in diversification rates relative to all the other groups).

To put this in context, up until ~20 million years ago, australia was largely wet and tropical/subtropical, but it has become much more arid over the past 20 mya (today it is 70% arid). The ancestor of this skink group was almost certainly adapted to a wet-tropical environment. It turns out that, as the climate became more arid, only a tiny handful of skinks were able to switch from wet-tropical to arid-adapted ecologies. This is surprising, because an enormous number of skink species inhabit the arid regions - but these are mostly species descended from just a tiny number that actually made the evolutionary transition necessary to thrive in the desert. The main question that comes out of this work is the one you ask about a genetic basis: we don't yet know much about why only a few lineages have been able to make a 'wet to arid' switch, but it is clearly not easy to do or many more species would have made this evolutionary transition. My hypothesis is that it involves physiological adaptations, and there is some evidence of this in the literature and even more anecdotal evidence, but this question really needs some serious on-the-ground comparative physiological analysis of a diverse range of skink species.

Originally Posted by ominously
> Sounds great, but all there is, is a flowery discussion of how they've
> diversified in direct opposition to historical trends. I would
> personally
> like to know the basis of their findings with more than observational
> evidence (not that this isn't interesting of and in itself mind you).

Depends on what you mean by observational evidence, I guess. All data in science is ultimately observational, whether it is computer-logged observations of subatomic collisions in a particle accelerator, carbon dioxide profiles from ancient antarctic ice cores, or population densities of humpback whales in the North Pacific. In our case, the observational data consist of DNA sequences (and the evolutionary relationships inferred from those sequences), as well as the numbers of species in different skink lineages.

Originally Posted by ominously
> Is this part of the gentic bar code studies? Are these 252 species
> genteically of geographically distinct from on another?"

This study is similar to the bar code studies you've heard about, because we also sequenced some mitochondrial DNA from the lizards. The DNA barcoding project uses the mitochondrial gene cytochrome C oxidase, but we used different genes. The 252 species are not geographically distinct from one another - some are separated from others, but you find regions where many species occur together. Much of my field research is done in the Great Victoria Desert in western Australia, where more than 20 of these skinks can be find living together in the same geographic region. All the species are genetically distinct from one another - certainly all those we've looked at.

Dan Rabosky
Department of Ecology and Evolutionary Biology &
Fuller Evolutionary Biology Program
Cornell Lab of Ornithology
Cornell University

Dan's research is really exciting, and I can't wait to see more of it published. Kudos to Dan for taking the time to respond to questions.

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